Gel underfill layers for light emitting diodes and methods of fabricating same

ABSTRACT

A light emitting device is fabricated by providing a mounting substrate and an array of light emitting diode dies adjacent the mounting substrate to define gaps. A gel that is diluted in a solvent is applied on the substrate and on the array of light emitting dies. At least some of the solvent is evaporated so that the gel remains in the gaps, but does not completely cover the light emitting diode dies. For example, the gel substantially recedes from the substrate beyond the array of light emitting diode dies and also substantially recedes from an outer face of the light emitting diode dies. Related light emitting device structures are also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/160,793, filed Jun. 15, 2011, entitled Conformal Gel Layersfor Light Emitting Diodes and Methods of Fabricating Same, assigned tothe assignee of the present application, the disclosure of which ishereby incorporated herein by reference as if set forth in its entiretyherein.

BACKGROUND OF THE INVENTION

This invention relates to light emitting devices and assemblies andmethods of manufacturing the same, and more particularly, to LightEmitting Diodes (LEDs) and assemblies thereof.

LEDs are widely known solid-state lighting elements that are capable ofgenerating light upon application of voltage thereto. LEDs generallyinclude a diode region having first and second opposing faces, andinclude therein an n-type layer, a p-type layer and a p-n junction. Ananode contact ohmically contacts the p-type layer and a cathode contactohmically contacts the n-type layer. The diode region may be epitaxiallyformed on a substrate, such as a sapphire, silicon, silicon carbide,gallium arsenide, gallium nitride, etc., growth substrate, but thecompleted device may not include a substrate. The diode region may befabricated, for example, from silicon carbide, gallium nitride, galliumphosphide, aluminum nitride and/or gallium arsenide-based materialsand/or from organic semiconductor-based materials. Finally, the lightradiated by the LED may be in the visible or ultraviolet (UV) regions,and the LED may incorporate wavelength conversion material such asphosphor.

LEDs are increasingly being used in lighting/illumination applications,with a goal being to provide a replacement for the ubiquitousincandescent light bulb.

SUMMARY OF THE INVENTION

Light emitting devices according to various embodiments described hereininclude a mounting substrate and an array of light emitting diode diesadjacent the mounting substrate, to define a gap between a respectivelight emitting diode die and the mounting substrate that is adjacentthereto. A gel layer is provided on the mounting substrate. The gellayer extends at least partially in the gaps between the light emittingdiode dies and the substrate adjacent thereto, to provide an underfill.However, the gel layer does not completely cover the light emittingdiode dies. In some embodiments, the gel layer does not extend on thesubstrate substantially beyond the array of light emitting diode dies,and also does not extend substantially on an outer face of the lightemitting diode dies. As used herein, a “gel” means a colloid in which adisperse phase has combined with a dispersion medium to produce asemi-solid material, such as a jelly. Moreover, as used herein,“substantially” means that there may be some isolated islands of gelpresent in the given region, but the gel layer itself does not extend inthe given region.

In some embodiments, an array of bond regions is provided, a respectiveone of which connects a respective light emitting diode die to themounting substrate. A respective bond region is recessed from an edge ofa respective light emitting diode die, to define a respective gap.Moreover, in some embodiments, the gel layer fills the gaps. Also, insome embodiments, the gel layer also extends between adjacent lightemitting diode dies in the array. Finally, in other embodiments, the gellayer may also extend at least partially, and in some embodiments fully,on sidewalls of the light emitting diode dies.

Light emitting devices that include a gel underfill layer according toany embodiments described herein, may also include a conformal ornonconformal phosphor layer. As used herein, a “conformal” layerincludes opposing surfaces that both conform to a contour of theunderlying element on which the conformal layer extends. Moreover, asused herein, the term “phosphor” includes any wavelength conversionmaterials that are sometimes called luminescent, fluorescent and/orphosphorescent. The phosphor layer is directly on the substrate, extendsbeyond the array of light emitting diode dies and also extends directlyon the outer faces of the light emitting diode dies. In otherembodiments, a lens is provided that spans the array of light emittingdiode dies and extends directly on the substrate beyond the array oflight emitting diode dies and directly on the outer faces of the lightemitting diode dies. A conformal or nonconformal phosphor layer and alens may also be provided according to other embodiments describedherein, wherein the lens is on at least a portion of the phosphor layer.

Various embodiments described above have included an array of lightemitting diode dies adjacent the mounting substrate. However, in otherembodiments, a single light emitting diode die may be provided adjacentthe mounting substrate to define a gap between the light emitting diodedie and the mounting substrate that is adjacent thereto. A gel layer isprovided on the mounting substrate. The gel layer extends at leastpartially in the gap between the light emitting diode die and thesubstrate adjacent thereto, but does not completely cover the lightemitting diode die. In some embodiments, the gel does not extend on thesubstrate substantially beyond the light emitting diode die, and alsodoes not extend substantially on an outer face of the light emittingdiode die. In some embodiments, the gel layer fills the gap and/orextends at least partially on sidewalls of the light emitting diode die,as also described above. A phosphor layer and/or a lens may also beprovided, as was described above.

Light emitting devices may be fabricated, according to variousembodiments described herein, by providing a mounting substrate and anarray of light emitting diode dies adjacent the mounting substrate todefine a gap between a respective light emitting diode die and themounting substrate that is adjacent thereto. A gel that is diluted in asolvent is applied on the substrate and on the array of light emittingdiode dies. At least some of the solvent is then evaporated, so that thegel remains in the gaps, but does not completely cover the lightemitting diode dies. In some embodiments, the gel substantially recedesfrom the substrate, for example from reflective areas of the substrate,beyond the array of light emitting diode dies, and also substantiallyrecedes from an outer face of the light emitting diode dies.

The gel may be applied by dispensing at least one drop of gel that isdiluted in the solvent on the substrate and on the array of lightemitting diode dies, and/or by spraying the gel that is diluted in thesolvent on the substrate and on the array of light emitting diode dies.Other gel-applying techniques also may be used. In some embodiments, thegel is diluted in the solvent at a ratio of about 1:4-5 by volume. Insome embodiments, the gel may fill the gaps. In other embodiments, thegel may remain in the gaps and on the substrate between adjacent lightemitting diode dies, but substantially recedes from the substrate beyondthe array of light emitting diode dies and also substantially recedesfrom an outer face of the light emitting diode dies. In yet otherembodiments, the gel may remain at least partially on sidewalls of thelight emitting diode dies, as well. A phosphor layer and/or a lens maythen be applied or attached.

Method embodiments described above have included an array of lightemitting diode dies. However, in other methods described herein, asingle light emitting diode die may be provided adjacent the mountingsubstrate to define a gap, and a gel that is diluted in solvent isapplied on the substrate and on the light emitting diode die. At leastsome of the solvent is evaporated so that the gel remains in the gap,but does not completely cover the light emitting diode die. In someembodiments, the gel substantially recedes from the substrate, forexample from reflective areas of the substrate, beyond the lightemitting diode die, and also substantially recedes from an outer face ofthe light emitting diode die. The gel may be applied using dispensing,spraying and/or other techniques, and the gel may fill the gap and/orremain at least partially on sidewalls of the light emitting diode dieas was described above. Phosphor and/or a lens also may be applied orattached as was described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views of light emitting devices thatmay be used with various embodiments described herein.

FIGS. 3-7 are cross-sectional views of light emitting devices accordingto various embodiments described herein.

FIG. 8 is a flowchart of methods of fabricating light emitting devicesaccording to various embodiments described herein.

FIGS. 9A-9B are photographs illustrating results of reflow operationsfor light emitting devices that do not use a conformal gel layeraccording to various embodiments described herein, and FIGS. 9C-9D arephotographs of the same light emitting devices that do use a conformalgel layer according to various embodiments described herein.

FIG. 10 is a flowchart of methods of fabricating light emitting devicesaccording to various embodiments described herein.

FIGS. 11-19 are cross-sectional views of light emitting devicesaccording to various embodiments described herein, during intermediateor final fabrication according to various embodiments described herein.

FIGS. 20-22 are optical microscope images of light emitting devicesaccording to various embodiments described herein.

FIGS. 23-25 are Scanning Electron Microscope (SEM) images of lightemitting devices according to various embodiments described herein.

FIGS. 26-27 are Electron Diffraction Spectroscopy (EDS) spectra of lightemitting devices according to various embodiments described herein.

DETAILED DESCRIPTION

The present invention now will be described more fully with reference tothe accompanying drawings, in which various embodiments are shown. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art. In the drawings, the size and relative sizes oflayers and regions may be exaggerated for clarity. Like numbers refer tolike elements throughout.

It will be understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. Furthermore, relative terms such as “beneath” or “overlies” maybe used herein to describe a relationship of one layer or region toanother layer or region relative to a substrate or base layer asillustrated in the figures. It will be understood that these terms areintended to encompass different orientations of the device in additionto the orientation depicted in the figures. Finally, the term “directly”means that there are no intervening elements. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items and may be abbreviated as “/”.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother region, layer or section. Thus, a first element, component,region, layer or section discussed below could be termed a secondelement, component, region, layer or section without departing from theteachings of the present invention.

Embodiments of the invention are described herein with reference tocross-sectional and/or other illustrations that are schematicillustrations of idealized embodiments of the invention. As such,variations from the shapes of the illustrations as a result, forexample, of manufacturing techniques and/or tolerances, are to beexpected. Thus, embodiments of the invention should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result, for example, frommanufacturing. For example, a region illustrated or described as arectangle will, typically, have rounded or curved features due to normalmanufacturing tolerances. Thus, the regions illustrated in the figuresare schematic in nature and their shapes are not intended to illustratethe precise shape of a region of a device and are not intended to limitthe scope of the invention, unless otherwise defined herein.

Unless otherwise defined herein, all terms (including technical andscientific terms) used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. It will be further understood that terms, such as those definedin commonly used dictionaries, should be interpreted as having a meaningthat is consistent with their meaning in the context of the relevant artand this specification and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Some embodiments now will be described generally with reference togallium nitride (GaN)-based light emitting diodes on silicon carbide(SiC)-based growth substrates for ease of understanding the descriptionherein. However, it will be understood by those having skill in the artthat other embodiments of the present invention may be based on avariety of different combinations of growth substrate and epitaxiallayers. For example, combinations can include AlGaInP diodes on GaPgrowth substrates; InGaAs diodes on GaAs growth substrates; AlGaAsdiodes on GaAs growth substrates; SiC diodes on SiC or sapphire (Al₂O₃)growth substrates and/or a Group III-nitride-based diode on galliumnitride, silicon carbide, aluminum nitride, sapphire, zinc oxide and/orother growth substrates. Moreover, in other embodiments, a growthsubstrate may not be present in the finished product. For example, thegrowth substrate may be removed after forming the light emitting diode,and/or a bonded substrate may be provided on the light emitting diodeafter removing the growth substrate. In some embodiments, the lightemitting diodes may be gallium nitride-based LED devices manufacturedand sold by Cree, Inc. of Durham, N.C.

Light Emitting Devices

FIGS. 1-2 provide detailed embodiments of representative light emittingdevices that may be used with conformal gel layers according to variousembodiments that will be described herein. However, many otherembodiments of light emitting devices may be used with conformal gellayers, as will be described in detail below. FIGS. 1 and 2 correspondto FIGS. 1 and 2 of application Ser. No. 13/112,502 to Emerson et al.,entitled “Gap Engineering for Flip-Chip Mounted Horizontal LEDs”, filedMay 20, 2011, assigned to the assignee of the present application, thedisclosure of which is hereby incorporated herein by reference in itsentirety as if set forth fully herein.

FIG. 1 is a cross-sectional view of a light emitting diode die and apackaged light emitting device. Referring to FIG. 1, the light emittingdiode die 100 includes a diode region 110 having first and secondopposing faces 110 a, 110 b, respectively, and including therein ann-type layer 112 and a p-type layer 114. Other layers or regions may beprovided, which may include quantum wells, buffer layers, etc., thatneed not be described herein. An anode contact 160 ohmically contactsthe p-type layer 114 and extends on a first face 110 a. The anodecontact 160 may directly ohmically contact the p-type layer 114, or mayohmically contact the p-type layer 114 by way of one or more conductivevias 162 and/or other intermediate layers. A cathode contact 170ohmically contacts the n-type layer 112 and also extends on the firstface 110 a. The cathode contact may directly ohmically contact then-type layer 112, or may ohmically contact the n-type layer 112 by wayof one or more conductive vias 172 and/or other intermediate layers. Asillustrated in FIG. 1, the anode contact 160 and the cathode contact 170that both extend on the first face 110 a are coplanar. The diode region110 also may be referred to herein as an “LED epi region”, because it istypically formed epitaxially on a substrate 120. For example, a GroupIII-nitride based LED epi 110 may be formed on a silicon carbide growthsubstrate. In some embodiments, the growth substrate may be present inthe finished product. In other embodiments, the growth substrate may beremoved. In still other embodiments, another substrate may be providedthat is different from the growth substrate, and the other substrate maybe bonded to the LED after removing the growth substrate.

As also shown in FIG. 1, a transparent substrate 120, such as atransparent silicon carbide growth substrate or a transparent sapphiregrowth substrate, is included on the second face 110 b of the dioderegion 110. The transparent substrate 120 includes a sidewall 120 a andmay also include an inner face 120 c adjacent the second face 110 b ofthe diode region 110 and an outer face 120 b, remote from the inner face120 c. The outer face 120 b is of smaller area than the inner face 120c. In some embodiments, the sidewall 120 a may be stepped, beveledand/or faceted, so as to provide the outer face 120 b that is of smallerarea than the inner face 120 c. In other embodiments, as shown in FIG.1, the sidewall is an oblique sidewall 120 a that extends at an obliqueangle θ, and in some embodiments at an obtuse angle, from the outer face120 b towards the inner face 120 c. Non-oblique sidewalls andapproximately equal faces 120 b and 120 c also may be provided.

Various embodiments of phosphor layers 140 and diode regions 110 may beprovided. For example, in some embodiments, the diode region 110 isconfigured to emit blue light, for example light having a dominantwavelength of about 450-460 nm, and the conformal layer comprises yellowphosphor, such as YAG:Ce phosphor having a peak wavelength of about 550nm. In other embodiments, the diode region 110 is configured to emitblue light upon energization thereof, as described above, and theconformal layer 140 may comprise a mixture of yellow phosphor and redphosphor, such as a CASN-based phosphor.

Continuing with the description of FIG. 1, the LED die 100 may becombined with a mounting substrate, such as a submount 180, and a lens190 to provide a light emitting device 200. The submount 180 may includea body 182 that may comprise aluminum nitride (AlN). In otherembodiments, metal core substrates, printed circuit boards, lead framesand/or other conventional packaging substrates may be used to mount theLED die 100 in a flip-chip configuration. The submount 180 includes asubmount face 182 a, and an anode pad 184 and a cathode pad 186 thereon.The anode and cathode pads may comprise silver-plated copper and/orother conductive materials. As illustrated in FIG. 1, the LED die 100 ismounted on the submount 180, such that the first face 110 a is adjacentthe submount face 182 a, the outer face 110 b is remote from thesubmount 180, the anode contact 184 is adjacent the anode pad 160, andthe cathode contact 186 is adjacent the cathode pad 170. In someembodiments, a bonding layer, such as a eutectic gold/tin solder layer188, is used to electrically, thermally and mechanically connect theanode contact 160 to the anode pad 184, and the cathode contact 170 tothe cathode pad 186. In other embodiments, direct attachment of theanode contact 160 to the anode pad 184, and direct attachment of thecathode contact 170 to the cathode pad 186 may be provided, for exampleusing thermocompression bonding and/or other techniques.

A packaged device anode 192 and a packaged device cathode 194 may beprovided on a second face 182 b of the submount body 182, and may beconnected to the anode pad 184 and cathode pad 186, respectively, usinginternal vias and/or conductive layers that extend on and/or around thesubmount body 182.

Finally, the packaged light emitting device 200 may also include a lens190 that extends from submount face 180 a to surround the LED die 100.The lens 190 may be a molded plastic lens.

FIG. 2 is a cross-sectional view of another LED die and packaged lightemitting device. Compared to embodiments of FIG. 1, the phosphor layer140′ extends across the diode region 110 and/or on the first face 182 ofthe submount body 182. Moreover, as shown in FIG. 2, the submount 180may include a layer 194 on the first face 182 a thereof. The layer 194may be an extension of the anode pad 184 and the cathode pad 186 or maybe distinct therefrom. In some embodiments, the layer 194 is areflective layer that extends between the submount face 182 a and theconformal layer 140′ that includes phosphor that extends on the submountface 182 a. This reflective layer 194 can reflect light that passesthrough the phosphor layer that is on the submount face 182 a backtoward the lens 190, and can thereby increase efficiency of the LED.

Packaged light emitting devices as described above in connection withFIGS. 1 and 2 may be embodied as a Cree® XLamp® XP-E High-EfficiencyWhite (HEW) LED, as described, for example, in the Cree® XLamp® XP-EHigh-Efficiency White LEDs Data Sheet, Publication No. CLD-DS34, Rev. 0,dated Dec. 6, 2010, and available at cree.com/products/xlamp_xpe.asp,the disclosure of which is hereby incorporated herein by reference inits entirety as if set forth fully herein.

FIGS. 1-2 illustrate light emitting diode dies that are configured forflip-chip mounting on a mounting substrate. Various configurations offlip-chip mounted light emitting diode dies may be used in variousembodiments described herein. Other light emitting devices according tovarious embodiments described herein may be configured for non-flip-chipmounting on a mounting substrate, as described and illustrated, forexample, in U.S. Patent Application Publication 2011/0031502 to Bergmannet al. entitled “Light Emitting Diodes Including Integrated BacksideReflector and Die Attach”, filed Aug. 10, 2009, assigned to the assigneeof the present application, the disclosure of which is herebyincorporated herein by reference in its entirety as if set forth fullyherein. Moreover, other light emitting devices according to variousembodiments described herein may be configured as vertical lightemitting devices, as described and illustrated, for example, in U.S.Pat. No. 6,791,119 to Slater, Jr et al., entitled “Light Emitting DiodesIncluding Modifications for Light Extraction”, filed Jan. 25, 2002,assigned to the assignee of the present application, the disclosure ofwhich is hereby incorporated herein by reference in its entirety as ifset forth fully herein.

Conformal Gel Layers for Light Emitting Diodes

Various embodiments of gel layers as described herein may be used withhorizontal light emitting diode dies, in which both the anode andcathode contacts are on the same surface of the die, or vertical lightemitting diode dies, in which the anode and cathode contacts are onopposite surfaces of the light emitting diode die. Moreover, gel layersaccording to various embodiments described herein may be used withhorizontal LED dies that are mounted on a mounting substrate in aflip-chip (contacts adjacent the mounting substrate) or non-flip-chip(contacts remote from the mounting substrate) orientation. Similarly,for vertical devices, gel layers may be used for light emitting diodedies that are mounted on a mounting substrate with the anode contactadjacent the mounting substrate and the cathode contact remote from themounting substrate, or with the cathode contact adjacent the mountingsubstrate and the anode contact remote from the mounting substrate. Yetother configurations of LED dies may be used.

Various embodiments described herein may arise from a recognition that athin and/or conformal gel layer may be used to increase the reliabilityand/or provide other potential advantages when packaging light emittingdiode dies on a mounting substrate. In some embodiments, the thin and/orconformal gel layer may be used to at least partially fill a gap betweenone or more bond regions that connects the light emitting diode die tothe mounting substrate and the edge of the light emitting diode die, soas to prevent high modulus silicone that is used in the phosphor layerfrom entering into the gap under and around the perimeter of the lightemitting diode die sufficiently to degrade the reliability of the bondbetween the light emitting diode die and the mounting substrate.Moreover, the low modulus thin and/or conformal gel layer may also beemployed as a shock absorber or a buffer layer between the phosphorcoating and/or lens and the LED die, to reduce stress on the die attachmetal bonds during high temperature reflow and/or subsequent deviceoperation. As used herein, “modulus” means an elastic modulus or acomplex shear modulus, commonly referred to as G*. The complex shearmodulus includes more than one component, such as a storage modulus anda loss modulus. In the case of shear loading, it may also be referred toas a “dynamic modulus”.

The gel layer according to various embodiments described herein may becontrasted with a glob of gel or other encapsulation material that maybe applied to LEDs as described, for example, in U.S. Pat. No. 6,590,235to Carey et al., entitled “High Stability Optical Encapsulation andPackaging for Light-Emitting Diodes in the Green, Blue, and Near UVRange”. Specifically, a glob of gel that is applied to an LED istypically quite thick. In contrast, a gel layer according to someembodiments described herein may be quite thin and may be less thanabout 20 μm thick in some embodiments, less than about 10 μm thick inother embodiments, and about 5 μm or about 3 μm thick in still otherembodiments. The gel layer may also be a conformal gel layer in someembodiments. Moreover, a phosphor layer may be provided on the gel layeraccording to some embodiments described herein. In some embodiments, thegel layer and the phosphor layer both comprise silicone, but thephosphor layer is not a gel. In other embodiments, the gel layer is atleast five times thinner than the phosphor layer. In still otherembodiments, the gel layer has a modulus that is less than the phosphorlayer and, in some embodiments, that is at least one, two or threeorders of magnitude less than the phosphor layer.

It would also not be predictable to apply a conformal gel layer on themounting substrate that extends at least partially into the gap, becausethe silicone-containing phosphor coating or silicone-containing moldedlens that are typically used may already be regarded as providing stressrelief. However, these silicone-containing coatings may have arelatively high modulus, and may degrade the package reliability duringreflow or subsequent device operation, if they encroach into the gap. Incontrast, by incorporating a conformal gel layer in the gap, thephosphor layer or molded lens may be prevented from encroaching into thegap. Moreover, the gel layer may exhibit sufficiently low modulus so asnot to degrade reliability during reflow and/or subsequent deviceoperation. In other embodiments, the conformal gel layer can be appliedto light emitting diode dies in a cavity package and/or a planar packagethat uses a silicone dam to contain the phosphor layer, with thephosphor layer dispensed to fill the cavity around the gel-coated LEDdie. The low modulus of the gel layer can provide stress relief to theLED die, so as not to degrade reliability during reflow and/orsubsequent device operation.

It would also not be predictable to use a conformal gel layer on an LEDdie between a phosphor layer and the LED die. Specifically, it is knownthat the LED die can provide heat sinking for the phosphor layer, andthat it is desirable to anchor the phosphor layer to the LED die. Yet,an intermediate conformal gel layer between a conformal or non-conformalphosphor layer and the LED die can actually retain the heat sinkingabilities of the LED die while enhancing the reliability of the device,by providing a buffer layer. Moreover, by being conformal to the LEDdie, anchoring of the phosphor layer and/or the lens may still takeplace.

FIG. 3 is a cross-sectional view of a horizontal, flip-chip lightemitting device, such as was illustrated in FIGS. 1 and 2, according tovarious embodiments described herein. These light emitting devices 300may include a mounting substrate 310 and a light emitting diode die 320adjacent the mounting substrate, to define a gap G therebetween. Themounting substrate may correspond to the submount 180 and the lightemitting die may correspond to the LED die 100 of FIGS. 1 and 2 in someembodiments. More specifically, a bond region 330 that may be formed byan anode and/or a cathode contact of the LED die 320 (for example, asshown by contacts 160 and/or 170 of FIGS. 1 and 2), an anode and/or acathode pad of the mounting substrate 310 (for example, as shown by pads184 and/or 186 of FIGS. 1 and 2), and/or a bond layer therebetween (forexample, as shown by element 188 of FIGS. 1 and 2), is recessed from anedge of the LED die 320 to define the gap G. The bond region 330 maytherefore provide mechanical connection and may also provide one or moreelectrical connections. A conformal gel layer 340 is provided on themounting substrate 310 and includes opposing surfaces that both conformto a contour of the mounting substrate 310. The conformal gel layer 340extends at least partially into the gap G. In embodiments of FIG. 3, theconformal gel layer 340 only partially fills the gap G. However, inother embodiments, the conformal gel layer 340 may entirely fill the gapG and, in still other embodiments, the conformal gel layer 340 may bethicker than the height of the gap G. In some embodiments, the conformalgel layer 340 is less than about 20 μm thick. In other embodiments, itis less than about 10 μm thick and, in still other embodiments, it isabout 5 μm or about 3 μm thick. In other embodiments, the gel layer 340need not be conformal when it is so thin.

As was described above, the conformal gel layer 340 includes opposingsurfaces that both conform to a contour of the mounting substrate 310and/or the LED die 320. It will be understood that the conformal gellayer 340 need not conform to the contour of the mounting substrateand/or LED die on the entire surface thereof. For example, the conformalgel layer may be patterned so that it is not present on a portion of themounting substrate/LED die or may be non-conformal on a portion of themounting substrate/LED die. However, the conformal gel layer includesopposing surfaces that both conform to the contour of the mountingsubstrate/LED die over at least a portion thereof, for example adjacentthe LED die in some embodiments, and in other embodiments surroundingthe LED die.

Still referring to FIG. 3, in some embodiments, one or more conformalphosphor layers 350 are provided on the conformal gel layer 340, and alens 360 may be provided or molded on the mounting substrate 310. Forexample, the phosphor layer 350 may convert at least some blue lightthat is emitted from a blue LED 320 into yellow light and/or red light,so as to provide the appearance of white light. In other embodiments,phosphor may be provided in the molded lens 360, in an encapsulationmaterial, as one or more conformal or non-conformal layers, or phosphorneed not be provided at all. As was described above in connection withthe conformal gel layer 340, the conformal phosphor layer 350 need notbe conformal over the entire contour of the mounting substrate or LEDdie.

The conformal gel layer 340 may provide a filler in the gap G that canprevent the phosphor layer 350 or the molded lens 360 from entering thegap G sufficiently to degrade operation of the light emitting device300.

In some embodiments, the conformal gel layer 340, the phosphor layer 350and the molded lens 360 may all comprise silicone. However, in someembodiments, the conformal phosphor layer 350 and molded lens 360 arenot gels. For example, a Dow Corning Specification Sheet entitled “DowCorning LED SOLUTIONS Lighting the way to advanced materials andsolutions”, Form No. 11-1679A-01, 2008, the disclosure of which ishereby incorporated herein by reference in its entirety as if set forthfully herein, describes various silicone-based gels, elastomers andresins that may be used for encapsulants, lens molding and overmolding.Normal refractive index and high refractive index materials may beprovided. As shown, although various hardnesses may be attributed to theelastomers and resins, the gels have hardness that is so low as to beunmeasurable. In other embodiments, the gel may have a hardness of lessthan about 30 on the Shore A scale. The gel may also have an opticaltransmission of greater than about 95% and, in some embodiments, greaterthan about 98%, in the visible spectrum and a refractive index ofgreater than about 1.4 and, in some embodiments, greater than about 1.5.

As is well known to those having skill in the art, the hardness ofplastics is commonly measured by a Shore® (Durometer) test or Rockwellhardness test. Both tests measure the resistance of plastics towardindentation and provide an empirical hardness value that may notnecessarily correlate well to other properties or fundamentalcharacteristics. Shore Hardness, using either the Shore A or Shore Dscale, is generally used for rubbers/elastomers and is also commonlyused for “softer” plastics, such as polyolefins, fluoropolymers andvinyls. The Shore A scale is generally used for “softer” rubbers, whilethe Shore D scale is generally used for “harder” ones. Many other Shorehardness scales, such as Shore O and Shore H hardness, exist but areless frequently used. As described in the above-cited Dow CorningSpecification Sheet, the hardness of a gel is generally unmeasurableusing any conventional test or scale.

In addition to having significantly lower hardness, a silicone-based gelmay also have a significantly lower modulus compared to silicone-basedelastomers and resins that were described above. The significantly lowermodulus may be a primary factor in the ability of the gel to reduce orminimize stress on the light emitting diode die. For example, a DowCorning silicone-based gel as described above may have a complex shearmodulus (G*) of about 0.009 MPa at 25° C., while a comparable complexmodulus for a silicone-based elastomer or resin may be about 12 MPa at25° C. Thus, a silicone-based gel may have a complex modulus that islower than that of a silicone-based elastomer or resin, and in someembodiments at least about one, two or three orders of magnitude lowerthan that of a silicone-based elastomer or resin. While the precedingvalues were quoted at 25° C., the complex modulus behavior of the gelcontinues to be orders of magnitude lower than the elastomers or resinsover wide temperature ranges. Accordingly, conformal gel layers asdescribed herein may provide a low modulus layer that can provide bothhigher light output and better stress relief than elastomer-type orresin-type silicone formulations.

It will also be understood that the silicone-based gels described aboveare merely examples, and many other silicone or non-silicone-based gelsmay be used. In some embodiments of FIG. 3, the conformal gel layer 350may be selected from any of the gels, the phosphor layer may be selectedfrom any of the elastomers, and the molded lens may be selected from anyof the resins listed in the above-referenced Dow Corning SpecificationSheet.

FIG. 4 illustrates other embodiments of light emitting devices 400,wherein the conformal gel layer 340′ also extends from the gap G ontothe LED die 320 and, in some embodiments, covers the LED die 320. Inembodiments of FIG. 4, the phosphor layer 350′ is a conformal phosphorlayer. Accordingly, in embodiments of FIG. 4, the conformal gel layer340′ fills the gap G and also provides a buffer layer between theconformal phosphor layer 350′ and the LED die 320. In other embodiments,even if the gap G is not presented or is otherwise filled, the conformalgel layer 340′ may form a buffer layer or shock absorber between theconformal phosphor layer 350′ and the LED die 320. In other embodiments,phosphor may be provided in the molded lens 360, in an encapsulationmaterial, as one or more conformal or non-conformal layers, or phosphorneed not be provided at all.

In embodiments of FIG. 4, the conformal gel layer 340′ may besufficiently thin so as to not adversely degrade performance of thelight emitting device 400. More specifically, it is known that the LEDdie 320 can provide heat sinking to reduce degradation of the phosphorin the phosphor layer 350′. In some embodiments, the conformal gel layer340′ may be sufficiently thermally conductive and sufficiently thin soas to allow effective heat sinking to take place. In some embodiments,the conformal phosphor layer 350′ may be between about 50 μm and about100 μm thick and the conformal gel layer 340′ may be between about 3 μmand about 10 μm thick. In some embodiments, the gel layer is less thanabout 20 μm thick. In other embodiments, the gel layer is less thanabout 10 μm thick and, in still other embodiments, it is about 5 μm orabout 3 μm thick. Accordingly, the conformal gel layer 340′ may be atleast about five (5) times thinner than the conformal phosphor layer350′ in some embodiments. In other embodiments, the gel layer 340 neednot be conformal when it is so thin.

FIG. 5 is a cross-sectional view of a light emitting device according toother embodiments described herein. As shown in FIG. 5, these lightemitting devices 500 include a horizontal LED die 320′ that is mountedin non-flip-chip configuration, as described, for example, in theabove-cited U.S. Publication No. 2011/0031502, so that the anode andcathode contacts 332 and 334 are remote from the mounting substrate 310and a bond region 330 that may not provide electrical contact isprovided to mount the LED on the mounting substrate 310. A conformal gellayer 340″ is provided that extends from the mounting substrate 310 ontothe LED die 320′.

In FIG. 5, a gap G is not shown between the bond region 330 and the LEDdie 320. Yet, the conformal gel layer 340″ can provide potentialadvantages nonetheless. In particular, a conformal or non-conformalphosphor layer (not shown in FIG. 5) and/or the molded lens 360 mayexert upward (separating) forces on the LED die 320′, as shown by arrow512. This force may degrade the bond of the bond region 330 and maycause the LED die 320 to at least partially detach from the mountingsubstrate 310 during thermal cycling (reflow or subsequent operation) ormay cause the bond to degrade even if it remains attached. This forcemay be caused by the Coefficient of Thermal Expansion (CTE) of thephosphor layer and/or molded lens. The conformal gel layer 340′ mayprovide a buffer layer against this degradation, due to its low hardnessand low modulus. In other embodiments, phosphor may be provided in themolded lens 360, in an encapsulation material, as one or more conformalor non-conformal layers, or phosphor need not be provided at all.

FIG. 6 illustrates other embodiments of light emitting devices 600wherein a vertical LED die 320″ includes one of the anode or cathodecontact 610 adjacent the mounting substrate 310, and the other of theanode or cathode contact 620 remote from the mounting substrate 310, aswas illustrated, for example, in the above-cited U.S. Pat. No.6,791,119. A conformal gel layer 340′″ fills the gap G between thecontact 610 and the edge of the LED die 320′, and also provides a bufferlayer for the phosphor layer 350′. The phosphor layer 350′ may beconformal, semi-conformal (as illustrated) or non-conformal. In theseembodiments, the conformal gel layer 340′″ can at least partially fillthe gap G, if present, and can also reduce or prevent the LED die fromlifting off due to forces generated by the thermal expansion of thephosphor layer 350′ or the dome 360. In other embodiments of FIGS. 5 and6, the sidewall walls of the LED die 320′/320″ may be orthogonal to thefaces thereof or may form an obtuse angle with the mounting substrate310.

FIG. 7 illustrates other embodiments of light emitting devices 700. Inthese embodiments, a plurality of LED die 320′″ are mounted on amounting substrate 310 using a bonding region 710 that may also provideone or more electrical contacts. Any of the embodiments of LED die thatare described herein may be used. A conformal gel layer 340″″ isprovided on the face of the mounting substrate 310 and on the LED dies320′″. A glob 350″ of phosphor is provided, as well as a molded lens360. Unexpectedly, in embodiments of FIG. 7, it has been found that theconformal gel layer 340″″ can promote adhesion of the glob 350″ ofphosphor and/or the lens 360 in the device 700, by acting as a shockabsorber against the effects of thermal expansion in the phosphor layer350″ and/or the lens 360. Also unexpectedly, the conformal gel layer340″″ may be more effective than a non-conformal gel layer, such as maybe provided by conventional encapsulant, by providing additionalgripping surface to which the phosphor layer 350″ and/or lens 360 mayadhere, and may also preserve the heat sinking abilities of the LED dies320″.

Various embodiments illustrated in FIGS. 3-7 and described herein canalso provide a light emitting device that comprises a light emittingdiode die and a gel layer that is less than about 20 μm thick on thelight emitting diode die. The gel layer may be a conformal layerincluding opposing surfaces that both conform to a contour of the lightemitting diode die. In other embodiments, the gel layer is less thanabout 10 μm thick and, in still other embodiments, the gel layer isabout 3 μm or about 5 μm thick. A phosphor layer may be provided on thegel layer.

Yet other embodiments illustrated in FIGS. 3-7 and described herein canalso provide a light emitting device that comprises a light emittingdiode die, a gel layer on the light emitting diode die and a phosphorlayer on the gel layer. The gel layer and the phosphor layer may bothcomprise silicone, but the phosphor layer is not a gel. Moreover, thegel layer may be at least five times thinner than the phosphor layer.Furthermore, the gel layer may have a modulus that is less than thephosphor layer and, in some embodiments, the gel layer has a modulusthat is at least one, two or three orders of magnitude less than thephosphor layer.

Fabrication

Methods of fabricating light emitting devices according to variousembodiments described herein will now be described. In particular,referring to FIG. 8 at Block 810, a mounting substrate having a lightemitting diode die mounted thereon is conformally gel coated using, forexample, various processes that will be described below. Then, referringto Block 820, if a conformal phosphor coating is to be provided,phosphor is conformally coated on the mounting substrate having a lightemitting diode die mounted thereon that was conformally gel coated inBlock 810. In other embodiments, a non-conformal phosphor coating may beapplied or no phosphor coating may be applied. Further processing isthen performed at Block 830, for example to mold a lens, etc. It will beunderstood that the operations of Blocks 820 and 830 may be performedout of order from that shown in FIG. 8, and one of these blocks may alsobe omitted. Accordingly, FIG. 8 illustrates methods of fabricating alight emitting device wherein a mounting substrate having a lightemitting diode die mounted thereon is conformally gel coated and thenthe gel-coated mounting substrate is phosphor-coated and/or a lens ismolded thereon.

Various techniques may be used to conformally gel coat the mountingsubstrate having a light emitting die mounted thereon (Block 810).Specifically, in some embodiments, a thin conformal gel coat may beapplied by diluting a gel with a solvent, such as xylene, which can thenevaporate off, leaving a thin conformal gel coating layer. A sprayprocess, a drop dispense and/or other process may be used. In a sprayprocess, the gel that is diluted in the solvent is sprayed onto themounting substrate having the light emitting diode die mounted thereon,such that at least some of the solvent evaporates. Multiple spray passesmay be used. In a dispensing process, the gel that is diluted in thesolvent may be dispensed onto the mounting substrate having the lightemitting diode die mounted thereon, and then at least some of thesolvent may be evaporated, for example by heating. Multiple dispensesmay be used. Other techniques of forming a conformal gel coating may beused.

Experimental Results

The following examples shall be regarded as merely illustrative andshall not be construed as limiting the invention.

FIG. 9A is a photograph of an array of 16 LED die on a mountingsubstrate. A lens was then molded on the mounting substrate of FIG. 9A,and the device was subject to a reflow in a standard reflow oven using atypical reflow profile with a 260° C. peak for lead-free solder attach.The lens was then removed. FIG. 9B illustrates the result after lensremoval. As shown, only five of the LED die remain attached to thesubstrate, so that eleven of the die came off upon removal of the lens,indicating severe degradation of the die attach.

FIG. 9C illustrates an identical device as FIG. 9A, except that aconformal gel layer of Dow OE-6450 gel, about 7 μm thick, was appliedbefore molding the lens. The device was then subject to fifteen (15)reflows in the standard reflow oven using the typical reflow profilewith a 260° peak for lead-free solder attach, and the lens was thenremoved. No die attach failures were found, as illustrated in FIG. 9C.FIG. 9D illustrates an identical device as FIG. 9A, except the conformalgel layer of FIG. 9C was coated with a conformal phosphor layer, about50 μm thick, prior to molding the lens. Again, fifteen (15) reflows inthe standard reflow oven using the typical reflow profile with a 260° C.peak for lead-free solder attach were performed prior to removing thelens. Again, no die attach failures were found. Accordingly, a conformalgel layer according to various embodiments described herein can improvethe reliability of a light emitting device and reduce or prevent dieattached degradation due to subsequent processing and/or deviceoperation.

Additional Discussion

Additional discussion of various embodiments described herein will nowbe provided. Various embodiments described herein can provide athermally robust LED package that may, in some embodiments, usehorizontal LED dies that are mounted on mounting substrates, such thatthe anode and cathode contacts are adjacent the mounting substrate. Athin conformal gel coat is applied to the die or die arrays by, forexample, diluting the gel with xylene, which evaporates off, leaving avery thin layer. The gel coat may be applied with a heated spray processor a drop dispense of highly diluted silicone (for example, five partsxylene by volume to one part silicone) followed by heating. Coating canbe performed, for example, prior to phosphor deposition for white lightemitting devices or just prior to lens molding for blue light emittingdevices. The gel can provide a buffer to the harder silicones used forthe phosphor coating or lens molding, to reduce or minimize stress onthe die attach metal bonds during temperature reflow and/or packageoperation.

For applications where the thin, conformal gel coating is applied underthe conformal phosphor layer, the thin conforming gel coating maymaintain the conformal phosphor layer very close to the LED die surface,to thereby provide similar heat sinking as that of a part without thegel.

Conventionally, a glob or fill approach has been used to cover the LEDdie with a gel-like material under a harder lens material. Someembodiments described herein can provide similar benefits asconventional glob or fill gel, while allowing improved lens adhesion,particularly for small die arrays. In such small die arrays, thetopography may be maintained to provide better mechanical attachment ofthe phosphor layers and/or lens. It may also maintain the necessary heatsinking of the phosphor layer to reduce or prevent heating and charring.

Moreover, a gel layer according to various embodiments described hereinmay provide unexpected advantages for small LED dies. In particular, asthe die size is reduced from about 1 mm², the potential for negativeimpact on the die attach due to the silicone resin or elastomerproperties is increased, since the die attach area becomes increasinglysmaller. In addition to a reduction in the absolute die attach area, therelative die attach area as a percentage of total die area is alsogenerally reduced, as the electrode spacing, electrode setbacks from thedevice edges and/or street widths may follow the same design rules asthe larger dies. Accordingly, the ratios of the perimeter gap area andelectrode gap area under the die relative to the die attach areagenerally increase as the die is made smaller, exacerbating the impactof silicone resin or elastomer penetration in these areas given thereduction in absolute die attach area. Die sizes below about 0.25 mm²may be particularly sensitive. For example, on a 350×470 mm directattach die, the die attach pads represent about 58% of the total diearea. However, on the 240×320 mm direct attach die shown in FIGS. 9A-9D,the die attach area is reduced to about 48% of the total die area. Forreference, on a 1 mm² die, the die attach area is greater than about 80%of the die area. Accordingly, a gel layer according to variousembodiments described herein may be particularly useful as LED die sizedecreases.

Gel Underfill Layers

Various embodiments described above can provide conformal and/or thingel layers for light emitting devices. However, there may be otherapplications where it is desirable to restrict the gel to an underfilllayer in the gap(s) between the light emitting diode die(s) and themounting substrate that is adjacent thereto, without completely coveringthe LED die(s), e.g., without providing substantial amounts of gel onthe outer face of the LED die(s) and without providing substantialamounts of gel on the substrate substantially beyond the array of LEDdie(s). Thus, it may be desirable to provide a gel underfill at the baseof the LED die(s) that may extend at least partially onto the sidewallsof the LED die(s), but that does not extend on the substratesubstantially beyond the LED die(s) and also does not extendsubstantially on an outer face of the LED die(s). Leaving most of theouter surface of the LED die(s) substantially bare and leaving thesubstrate beyond the LED die(s) substantially bare can improve ormaximize thermal heat sinking and adhesion of a phosphor layer and/or anovermolded lens to the light emitting device.

FIG. 10 is a flowchart of fabricating a light emitting device accordingto various embodiments provided herein. A mounting substrate and one ormore LED die(s) adjacent the mounting substrate are provided, to definea gap between a respective LED die(s) and the mounting substrate that isadjacent thereto. As used herein, more than one LED die that is providedadjacent the mounting substrate is also referred to as an “array” of LEDdies.

Referring to Block 1010, a gel that is diluted in a solvent is appliedon the substrate and on the LED die(s). The gel may be applied bydispensing at least one drop of gel that is diluted in the solvent, byspraying the gel that is diluted in the solvent and/or by otherapplication techniques. In some embodiments, the gel may be diluted inthe solvent at a ratio of about 1:4-5 by volume.

Then, at Block 1020, at least some of the solvent is evaporated so thatthe gel remains in the gaps, but does not completely cover the LEDdie(s). In some embodiments, the gel substantially recedes from thesubstrate beyond the array of LED die(s), and also substantially recedesfrom an outer face of the LED die(s). A conformal or nonconformal layerof phosphor and/or a lens may then be applied and/or attached at Block1030, according to any of the techniques described above and/or usingother techniques.

Various embodiments described herein can provide a thermally robustlight emitting device. A gel underfill is formed at the base of the LEDdie(s) by applying a solution of highly diluted gel to the LED die(s).As the solvent is driven off by evaporation, surface tension and/orother forces pulls the gel substantially off the outer die face(s) andoff the mounting substrate substantially beyond the LED die(s), forminga gel underfill layer around the base of the LED die(s). The level offilling around and between the LED dies can be varied based on, forexample, the initial dispense volume and/or the ratio of solvent to gel.Thus, the gel does not completely cover the LED die(s).

By leaving the outer face(s) of the LED die(s) substantially bare and byleaving the substrate beyond the LED die(s) substantially bare,according to some embodiments described herein, improved or maximizedthermal heat sinking and adhesion of a phosphor layer and/or overmoldedlens may be provided. At the same time, the gel underfill layer canreduce or prevent the harder encapsulants typically used in the phosphorlayer and/or lens molding from penetrating in the gap under the LEDdie(s), to thereby reduce or minimize stress on the die attach metalbonds during high temperature reflow and/or device operation.

Accordingly, the underfill may be restricted to the base of die(s) andthe die face(s), and the substrate beyond the die(s) may be leftsubstantially bare, even though the gel solution is applied all over thesubstrate and the outer face(s) of the die(s). Given the LED die size,geometry and packing density, it may be very difficult to provideconventional underfill dispense around the perimeter of the LED die(s).However, with the outer face(s) substantially bare according to variousembodiments described herein, the phosphor and/or lens can be inintimate contact with the die(s) to increase or maximize thermal heatsinking and adhesion. The surface tension also can restrict the coatingarea to the die(s), leaving substantially no or minimal gel on thesubstrate face outside the die(s). This allows the phosphor layer orlens to attach to the substrate and outer die face(s), which can providesimilar lens attachment strength as a package with no gel underfill.

FIG. 11 is a cross-sectional view of a light emitting device accordingto various embodiments described herein, and may correspond to a lightemitting device after a gel is applied at Block 1010 of FIG. 10, butbefore evaporation at Block 1020 of FIG. 10.

Specifically, referring to FIG. 11, an array of LED dies 320′″ isprovided adjacent a mounting substrate 310 to define a gap G between arespective LED die 320′″ and the mounting substrate 310 that is adjacentthereto. The gap G may be established by a bonding region 710, as wasdescribed above. As shown in FIG. 11, one or more LEDs 320′″ may beprovided. The LEDs 320′″ may be identical or different therebetween. Forexample, as shown in FIG. 11, the LED sidewalls may be vertical(orthogonal), oblique (obtuse or acute) or beveled. A gel that isdiluted in a solvent 1110 is provided on the substrate 310 and on theLED die(s) 320′″. The gel that is diluted in a solvent may be appliedusing a heated or unheated spray process or a heated or unheated dropdispense of highly diluted silicone in a solvent. For example, aboutfour to five parts xylene by volume to about one part silicone gel maybe used. FIG. 11 illustrates a drop profile of the gel layer that isdiluted in solvent that may be obtained, for example, using dropdispensing. When spraying is used, a conformal, semiconformal ornonconformal gel layer that is diluted in a solvent may be produced.

FIGS. 12-15 are cross-sectional views of light emitting devicesaccording to various embodiments described herein, and may correspond tolight emitting devices after evaporating at least some of the solvent atBlock 1020 of FIG. 10. In all of the embodiments of FIGS. 12-15, atleast some of the solvent 1110 is evaporated so that the gel remains inthe gaps G, but does not completely cover the LED die(s) 320′″.

Referring now to FIG. 12, at least some of the solvent 1110 isevaporated, for example at room temperature or above, so that the gel1110 a remains in the gap(s) G, but substantially recedes from thesubstrate 310 beyond the LED die(s) 320′″ and also substantially recedesfrom an outer face of the LED die(s) 320′″. As also shown in FIG. 12,the gel may also fill the gap(s) G. Moreover, as also shown, the gel mayalso extend on the substrate 310 between adjacent LED dies 320′″ in thearray and may also extend fully onto sidewalls of the LED die(s) 320′″.

In FIG. 12, the gel layer 1110 a extends along the entire sidewalls ofthe LED die(s) 320′″. In contrast, in FIG. 13, the gel layer 1110 bextends only partially onto the sidewalls of the LED die(s) 320′″.Moreover, in FIG. 14, the gel layer 1110 c does not extend onto the LEDdie sidewalls at all. Finally, in FIG. 15, the gel layer 1110 d does notextend fully onto the substrate between adjacent LED dies 320′″.

Thus, during evaporation, the gel remains in the gap(s) G, butsubstantially recedes from the substrate 310 beyond the light emittingdiode die(s) and also substantially recedes from outer face(s) of theLED die(s) (FIGS. 12-15). The gel may also remain between adjacent LEDdie(s) (FIGS. 12-14) or may at least partially recede from between theadjacent LED die(s) (FIG. 14). The LED gel may also remain fully (FIG.12) or partially (FIG. 13) on the sidewalls of the LED die(s) or maysubstantially recede from the sidewalls of the LED die(s) (FIGS. 14-15).

The amount by which the gel recedes may be governed by many factors,including, but not limited to, the geometry and material compositions ofthe substrate, the LEDs and the bond regions; the composition, volumeand wettability of the gel relative to these regions; the compositionand volume of the solvent; the relative volumes of gel and solvent;and/or the evaporation conditions, such as time and temperature. Forexample, the gel may substantially recede from a reflective surface ofthe substrate, that comprise, for example, silver, aluminum and/orreflective ceramics such as Al₂O₃.

FIGS. 12-16 also illustrate light emitting devices according to variousembodiments described herein that include a mounting substrate 310 andone or more (an array) of LED dies 320′″ adjacent the mounting substrate310, to define a gap G between a respective LED die 320′″ and themounting substrate 310 that is adjacent thereto, wherein the gap G maybe defined by one or more bond regions 710. A gel layer 1110 a, 1110 b,1110 c and/or 1110 d is provided on the mounting substrate 310 thatextends at least partially in the gaps G between the LED die(s) 320′″and the substrate 310 adjacent thereto, but does not completely coverthe LED die(s) 320′″. In some embodiments, the gel layer does not extendon the substrate 310 substantially beyond the LED die(s) 320′″ and alsodoes not extend substantially on an outer face of the LED die(s) 320′″.The gel layer may fill the gaps, as shown in FIGS. 12-15; may extendbetween adjacent LED dies in the array, as shown in FIGS. 12-14; mayextend fully on sidewalls of the LED die(s), as shown in FIG. 12; mayextend partially on the sidewalls, as shown in FIG. 13; or may notextend substantially on the sidewalls, as shown in FIG. 15.

FIGS. 16-19 are cross-sectional views of LED devices according tovarious other embodiments described herein, after application ofphosphor and/or a lens at Block 1030 of FIG. 10. These figures are basedon the structure of FIG. 14. However, phosphor and/or a lens may beapplied to the structures of FIGS. 12-13 and/or 15 in other embodiments.

Referring to FIG. 16, a conformal or semiconformal phosphor layer 350 isapplied, for example using techniques described above, that extendsdirectly on the substrate 310 substantially beyond the LED die(s) anddirectly on the outer face(s) of the LED die(s). In FIG. 17, anonconformal phosphor layer 350″ is applied, for example usingtechniques described above, that extends directly on the substrate 310substantially beyond the LED die(s) and directly on the outer face(s) ofthe LED die(s). Alternatively, in FIG. 18, a conformal or semiconformalphosphor layer 350 may be applied that is directly on the substratesubstantially beyond the array of LED die(s) and directly on the outerface(s) of the LED die(s) 320′″, and a lens 360 may be applied, forexample by molding, on the phosphor layer 350. Finally, in FIG. 19, alens 360 is applied or molded on the substrate 310 substantially beyondthe LED die(s) 320′″ and directly on the outer face(s) of the LED die(s)320′″. Phosphor may be included in and/or on the lens 360.

FIGS. 20-22 are optical microscope images of light emitting devicesaccording to various embodiments described herein. FIG. 20 correspondsto Block 1010 of FIG. 10, wherein a wet, highly diluted 4:1 solution ofxylene:silicone gel is dispensed on top of a direct attach LED diearray. Surface tension keeps the fluid held tight to the array.

FIGS. 21 and 22 illustrate light emitting devices after evaporatingpursuant to Block 1020 of FIG. 10. Heat was applied, for example, atabout 150° C. for about 1-2 hours to drive off the solvent and cure thegel. In some embodiments, an initial one-hour cure may be provided atabout 150° C., followed by an additional one-hour cure at about 150° C.after phosphor and/or lens application. The solvent evaporation andreceding of the gel may take place in the first few minutes of theinitial cure. As shown in FIG. 21, the gel has substantially recededfrom the substrate beyond the array of LED die(s) (a discoloration isshown where the gel/solvent originally was present), and alsosubstantially recedes from the outer faces (tops) of the LED dies. Thegel remains in the gaps and also remains on the substrate betweenadjacent LED dies.

FIG. 22 is a more magnified optical microscope image showing theformation of the gel coating 1110 c around the bases of the LED dies andbetween the LED dies. As shown, the outer (top) surfaces of the LED diesand the remote (upper) surfaces of the sidewalls are substantially bare.

FIGS. 23-25 illustrate Scanning Electron Microscope (SEM) images oflight emitting devices according to various embodiments describedherein. Specifically, FIG. 23 is a tilted SEM image showing the gelformation around the bases of the dies, extending to the substrate andextending between the dies, with minimal amounts of gel seen on theremote (upper) surfaces of the sidewalls and on the outer faces (tops)of the dies. FIG. 24 is an analogous plan view of the SEM image showingthe gel between the dies with minimal amounts of gel on the uppersurfaces of the sidewalls and on the tops of the LED dies. FIG. 25 is ananalogous SEM image at a lower magnification, again showing the gelbetween the dies with minimal amounts of gel on the upper surfaces ofthe sidewalls and on the tops of the dies.

FIGS. 26-27 are Electron Diffraction Spectroscopy (EDS) spectra of lightemitting devices according to various embodiments described herein.Specifically, FIG. 26 is an EDS taken on the top (outer) face of the LEDdie. FIG. 26 illustrates a silicon carbide composition of the LED die,with a low carbon (C) peak that confirms minimal gel presence on theouter face. Note that the platinum (Pt) peak is from a thin conductivecoating that is applied for SEM purposes. In contrast, FIG. 27 is an EDSspectrum taken between the dies, to show an increased carbon peak fromthe presence of the silicone gel between the dies.

Without wishing to be bound by any theory of operation, surface tensionand/or other forces in a gel diluted in a solvent may be used to causeselective receding of the gel during evaporation of the solvent, so thatthe gel does not completely cover the LED dies and in some embodimentssubstantially recedes from the substrate beyond the array of LED dies,and also substantially recedes from outer faces of the LED dies, butremains in the gaps at the bases of the LED dies, and may also remain onthe substrate between adjacent LED dies and/or at least partially on theLED die sidewalls. The gel may thereby be blanket-applied by dropdispensing, spray coating and/or other blanket techniques, yet the finalproduct may only have gel where it is desired and may have substantiallyno gel in areas where it is not desired for thermal and/or structuralpurposes.

Many different embodiments have been disclosed herein, in connectionwith the above description and the drawings. It will be understood thatit would be unduly repetitious and obfuscating to literally describe andillustrate every combination and subcombination of these embodiments.Accordingly, the present specification, including the drawings, shall beconstrued to constitute a complete written description of allcombinations and subcombinations of the embodiments described herein,and of the manner and process of making and using them, and shallsupport claims to any such combination or subcombination.

In the drawings and specification, there have been disclosed embodimentsof the invention and, although specific terms are employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims.

1. A light emitting device comprising: a mounting substrate; an array oflight emitting diode dies adjacent the mounting substrate to define agap between a respective light emitting diode die and the mountingsubstrate that is adjacent thereto; and a gel layer on the mountingsubstrate that extends at least partially in the gaps between the lightemitting diode dies and the substrate adjacent thereto but does notcompletely cover the light emitting diode dies.
 2. A light emittingdevice according to claim 1 wherein the gel layer does not extend on thesubstrate substantially beyond the array of light emitting diode diesand also does not extend substantially on an outer face of the lightemitting diode dies.
 3. A light emitting device according to claim 1further comprising an array of bond regions, a respective one of whichconnects a respective light emitting diode die to the mountingsubstrate, a respective bond region being recessed from an edge of arespective light emitting diode die to define a respective gap.
 4. Alight emitting device according to claim 3 wherein the gel layer fillsthe gaps.
 5. A light emitting device according to claim 4 wherein thegel layer also extends between adjacent light emitting diode dies in thearray.
 6. A light emitting device according to claim 1 wherein the gellayer also extends between adjacent light emitting diode dies in thearray.
 7. A light emitting device according to claim 2 wherein the gellayer also extends at least partially on sidewalls of the light emittingdiode dies.
 8. A light emitting device according to claim 1 furthercomprising a phosphor layer directly on the substrate that extendsbeyond the array of light emitting diode dies and that also extendsdirectly on the outer faces of the light emitting diode dies.
 9. A lightemitting device according to claim 1 further comprising a lens thatspans the array of light emitting diode dies and that extends directlyon the substrate beyond the array of light emitting diode dies anddirectly on the outer faces of the light emitting diode dies.
 10. Alight emitting device according to claim 2 wherein the gel does notextend on reflective areas of the substrate substantially beyond thearray of light emitting diode dies.
 11. A light emitting devicecomprising: a mounting substrate; a light emitting diode die adjacentthe mounting substrate to define a gap between the light emitting diodedie and the mounting substrate that is adjacent thereto; and a gel layeron the mounting substrate that extends at least partially in the gapbetween the light emitting diode die and the substrate adjacent theretobut does not completely cover the light emitting diode die.
 12. A lightemitting device according to claim 11 wherein the gel layer does notextend on the substrate substantially beyond the light emitting diodedie and also does not extend substantially on an outer face of the lightemitting diode die.
 13. A light emitting device according to claim 11wherein the gel layer fills the gap.
 14. A light emitting deviceaccording to claim 12 wherein the gel layer also extends at leastpartially on sidewalls of the light emitting diode die.
 15. A lightemitting device according to claim 11 further comprising a phosphorlayer directly on the substrate beyond the light emitting diode die anddirectly on the outer face of the light emitting diode die.
 16. A lightemitting device according to claim 11 further comprising a lens thatspans the light emitting diode die and that extends directly on thesubstrate beyond the light emitting diode die and directly on the outerface of the light emitting diode die.
 17. A light emitting deviceaccording to claim 12 wherein the gel does not extend on reflectiveareas of the substrate substantially beyond the light emitting diodedie.
 18. A method of fabricating a light emitting device comprising:providing a mounting substrate and an array of light emitting diode diesadjacent the mounting substrate to define a gap between a respectivelight emitting diode die and the mounting substrate that is adjacentthereto; applying a gel that is diluted in a solvent on the substrateand on the array of light emitting diode dies; and evaporating at leastsome of the solvent so that the gel remains in the gaps but does notcompletely cover the light emitting diode dies.
 19. A method accordingto claim 18 wherein the evaporating comprises evaporating at least someof the solvent so that the gel remains in the gaps but substantiallyrecedes from the substrate beyond the array of light emitting diode diesand also substantially recedes from on an outer face of the lightemitting diode dies.
 20. A method according to claim 18 wherein theapplying comprises dispensing at least one drop of gel that is dilutedin the solvent on the substrate and on the array of light emitting diodedies.
 21. A method according to claim 18 wherein the applying comprisesspraying the gel that is diluted in the solvent on the substrate and onthe array of light emitting diode dies.
 22. A method according to claim18 wherein the gel is diluted in the solvent at a ratio of about 1:4-5by volume.
 23. A method according to claim 18 wherein the providingcomprises providing a mounting substrate, an array of light emittingdiode dies adjacent the mounting substrate and an array of bond regions,a respective one of which connects a respective light emitting diode dieto the mounting substrate, a respective bond region being recessed froman edge of a respective light emitting diode die to define a respectivegap.
 24. A method according to claim 19 wherein the evaporatingcomprises evaporating at least some of the solvent so that the gel fillsthe gaps but substantially recedes from the substrate beyond the arrayof light emitting diode dies and also substantially recedes from on anouter face of the light emitting diode dies.
 25. A method according toclaim 19 wherein the evaporating comprises evaporating at least some ofthe solvent so that the gel remains in the gaps and on the substratebetween adjacent light emitting diode dies but substantially recedesfrom the substrate beyond the array of light emitting diode dies andalso substantially recedes from on an outer face of the light emittingdiode dies.
 26. A method according to claim 19 wherein the evaporatingcomprises evaporating at least some of the solvent so that the gelremains in the gaps and at least partially on sidewalls of the lightemitting diode dies but substantially recedes from the substrate beyondthe array of light emitting diode dies and also substantially recedesfrom on an outer face of the light emitting diode dies.
 27. A methodaccording to claim 19 wherein the evaporating comprises evaporating atleast some of the solvent so that the gel remains in the gaps butsubstantially recedes from reflective areas of the substrate beyond thearray of light emitting diode dies and also substantially recedes fromon an outer face of the light emitting diode dies.
 28. A methodaccording to claim 18 wherein the evaporating is followed by applyingphosphor directly on the substrate beyond the array of light emittingdiode die and directly on the outer faces of the light emitting diodedies.
 29. A method according to claim 18 wherein the evaporating isfollowed by attaching a lens that spans the array of light emittingdiode dies and that extends directly on the substrate beyond the arrayof light emitting diode dies and directly on the outer faces of thelight emitting diode dies.
 30. A method of fabricating a light emittingdevice comprising: providing a mounting substrate and a light emittingdiode die adjacent the mounting substrate to define a gap between thelight emitting diode die and the mounting substrate that is adjacentthereto; applying a gel that is diluted in a solvent on the substrateand on the light emitting diode die; and evaporating at least some ofthe solvent so that the gel remains in the gap but does not completelycover the light emitting diode die.
 31. A method according to claim 30wherein the evaporating comprises evaporating at least some of thesolvent so that the gel remains in the gap but substantially recedesfrom the substrate beyond the light emitting diode die and alsosubstantially recedes from an outer face of the light emitting diodedie.
 32. A method according to claim 30 wherein the applying comprisesdispensing at least one drop of gel that is diluted in the solvent onthe substrate and on the light emitting diode die.
 33. A methodaccording to claim 30 wherein the applying comprises spraying the gelthat is diluted in the solvent on the substrate and on the lightemitting diode die.
 34. A method according to claim 31 wherein theevaporating comprises evaporating at least some of the solvent so thatthe gel fills the gap but substantially recedes from the substratebeyond the light emitting diode die and also substantially recedes fromon an outer face of the light emitting diode die.
 35. A method accordingto claim 31 wherein the evaporating comprises evaporating at least someof the solvent so that the gel remains in the gap and at least partiallyon sidewalls of the light emitting diode die but substantially recedesfrom the substrate beyond the array of light emitting diode die and alsosubstantially recedes from on an outer face of the light emitting diodedie.
 36. A method according to claim 31 wherein the evaporatingcomprises evaporating at least some of the solvent so that the gelremains in the gap but substantially recedes from reflective areas ofthe substrate beyond the light emitting diode die and also substantiallyrecedes from on an outer face of the light emitting diode die.
 37. Amethod according to claim 30 wherein the evaporating is followed byapplying phosphor directly on the substrate beyond the light emittingdiode die and directly on the outer face of the light emitting diodedie.
 38. A method according to claim 30 wherein the evaporating isfollowed by attaching a lens that spans the light emitting diode die andthat extends directly on the substrate beyond the light emitting diodedie and directly on the outer face of the light emitting diode die.